The present invention relates generally to machinery for manipulating electrodes and more particularly to devices and methods for joining graphite electrodes for use with metal arc furnaces.
Metal arc furnaces include large vessels for melting metal. Heat may be generated inside the furnaces using graphite electrodes across which electric current is passed. Heat is generated inside the furnace due to a high voltage arc formed by the current passing through one or more electrodes. The heat is used to melt metal.
During use, graphite electrodes are consumed within the furnace vessel, requiring electrodes to be replaced over time. As electrode material is consumed in the furnace, the electrode is shortened to a length where it is no longer independently usable. However, a partially-consumed electrode may be joined to a second partially-consumed or complete electrode to form a joined electrode that can be used. Thus, the partially-consumed electrode may still be useful when placed in combination with a second electrode portion.
Electrodes may be joined in an end-to-end configuration using a threaded joint in some applications. When electrodes are combined, electrical conductivity is generally maintained across the joint, and electricity may be passed from one electrode to the second in the joint electrode. A threaded joint is commonly used to join electrodes. The joining procedure requires at least one electrode to be rotated relative to a second electrode such that a threaded engagement occurs.
The joining process typically requires two stages. During a first stage, the first electrode is rotated relative to the second electrode to allow loose engagement of the corresponding threads. Rotation during this stage encounters relatively little resistance as the threads are rotating. This may be referred to as a spin-down rotation when a free electrode is spun about its longitudinal axis relative to a fixed electrode below the free electrode, to allow the threads to engage.
Before use, the threaded electrode joint must be tightened to a predetermined manufacturer's suggested torque value. A second stage of electrode joining occurs when the end walls of the two electrodes or other structures make contact, requiring greater force to torque the first electrode relative to the second electrode. During this stage, the torque applied generally increases as a function of angular position. This second stage may be referred to as a torque stage. Once a desired torque value, or a value within a desired torque range, is reached, force application is ceased and the electrodes are properly joined.
An improper torque application can damage threads, reduce electrical connectivity between the electrode sections, or cause failure of the joint. Such failure may be catastrophic where the separate electrode sections each weigh several hundred or even several thousand pounds. Because joined electrodes are typically moved inside an industrial setting using overhead transport rigging, joint failure can cause one or both electrode sections to crash down on equipment or workers, causing major damage or injury.
Conventional tools and methods for joining electrodes in a threaded joint include manually rotating one free electrode relative to a fixed electrode. This may be achieved using a manual wrench or other tool for grasping and rotating the free electrode. The applied torque may be measured manually using a torque gauge or manual torque wrench. The conventional manual joining technique is time-consuming for workers and may be dangerous in some applications due to the requirement that a worker be near the additional electrode operating the wrench. Additionally, manual procedures lead to variance in applied torque values as workers may not apply the same torque every time.
Others have attempted to solve the problems associated with manual joining of threaded electrodes by providing devices to assist in the torque application process. For example, U.S. Pat. No. 6,167,076 titled Electrode Wrench provides an apparatus for joining a threaded free electrode section with a threaded fixed electrode section for use in electric arc furnaces. The apparatus includes a driver and a plurality of pawls to grip the free electrode and turn it in one angular direction. The electrode wrench includes a pneumatic cylinder to apply torque against the free electrode in the second stage of joining. During use, a user may measure the pressure applied in the pneumatic cylinder and correlate the pressure to a range of torque values. Thus, a user may stop applying force once a pressure value corresponding to a desired torque range is reached. However, the electrode wrench typically does not provide independent verification, or feedback measurement of applied torque. The failure of such independent verification can lead to improper torque application on the free electrode and the undesirable and dangerous problems mentioned above.
What is needed then are electrode joining apparatuses with improved feedback torque detection.